This invention relates to an improved process for re-refining hydrocarbon oils. More specifically, this invention relates to a solvent refining process for reclaiming used lubricating oils. Still more specifically, this invention relates to a process for preparing quality lubricating oils from used waste lubricating and crankcase oils.
Shortages of petroleum have renewed attention to developing methods for conserving dwindling world supplies of crude oil until science and technology can close the gap with stimulated production, alternative energy sources and more efficient energy utilization. One approach to this problem has been to encourage better utilization of present supplies, which includes an estimated 1 billion gallons of used lubricating oil that is drained, dumped or burned each year in this country. These oils have generally been used as engine crankcase lubricants, transmission and gear oils and the like. Used lubricating oils commonly contain various additives such as detergents, antioxidants, corrosion inhibitors, and extreme pressure additives which are necessary for satisfactory performance, in addition to solid and liquid contaminants, some of which result from oxidation of the oil itself, and generally water and gasoline. Much of this oil could be recovered and reused if it were collected and if it could be effectively reprocessed. Instead, as much as one-third of this oil is indiscriminately dumped, contaminating both land and water. Much of the waste oil is burned and this, too, contributes to pollution of our environment by releasing metallic oxides from additives in the oil into the atmosphere.
A number of processes are available for the purification and reprocessing of lubricating oils. Often these processes involve the use of distillation followed by polishing or decolorizing treatment. However, to prevent coking and column fouling during distillation, some form of pretreatment is necessary to remove many of the additives and contaminants from the oil. Typically, the waste oil is first heated to drive off volatile hydrocarbons and water and then contacted with a strong mineral acid or, to a lesser extent, a caustic which precipitates out a large portion of the oil as sludge. The supernatant oil is separated from the sludge and neutralized with an acid or caustic as appropriate before distillation or other polishing or decolorizing treatment. A discussion of these and other re-refining methods is found in U.S. Bureau of Mines, Report of Investigations - RI 7884 (1974), Waste Lubricating Oil Research, An Investigation of Several Re-refining Methods.
However, the acid or caustic pretreatment processes have many disadvantages which render these processes undesirable. For example, in either process, a large percentage of the used oil is lost (up to 50%) creating large volumes of highly acidic or caustic sludge for which there is no known use and which is disposed of in a sanitary landfill or similar manner and may cause environmental pollution. The use of strong acids and caustics oftentimes alters the petroleum base composition of the lubricating oils, resulting in the loss of a substantial quantity of otherwise recoverably organic material ultimately resulting in a product deficient in properties required for high-quality lubricants. For example, the loss of higher molecular weight diaromatic and polyaromatic-polar materials may approach 70% on an original oil basis. These materials are generally associated with natural lubricity of the base oil and removal will adversely affect this parameter of the lubricant product. Likewise, the polar materials are responsible in part for natural resistance to oxidation and removal of these compounds contributes to the generally poor oxidation resistance of reprocessed lubricating oils. Both of these conditions can be overcome, to some extent, by the use of additives.
Other treatment processes have been developed, which attempt to meet the environmental objections of the previous processes. These processes utilize various liquid hydrocarbon diluents which may be combined with solvents such as alcohol or water-alcohol mixtures to form solvent precipitation solutions. A number of these solvent extraction systems were examined and reported upon in Bureau of Mines Report of Investigations RI 7925 (1974), Waste Lubricating Oil Research, Some Innovative Approaches to Reclaiming Used Crankcase Oil. While these processes do not cause a loss of the desirable aromatic compounds, neither are most of these processes effective in removing the contaminants from the waste oil and so must be combined with a more severe treatment which utilizes an acid or caustic in order to completely reprocess the waste oil.
A solvent precipitation process which effectively removes most of the additives and undesirable contaminants from used lubricating oil without destroying the natural lubricity and other desirable qualities of the base oil while providing high percentages of recovery is disclosed in copending U.S. patent application Ser. No. 734,838, filed Oct. 22, 1976 and assigned to the U.S. Energy Research and Development Administration. We have found that, by combining the pretreatment process described in the above patent application with additional, relatively mild process steps, we are able to prepare a preprocessed lubricating oil stock, which when combined with an appropriate package of additives, is able to meet or exceed the wear and lubrication standards which have been set by the automobile industry for lubricating oils.